1. Field of the Invention
The present invention relates to a semiconductor laser device of RWG (ridge waveguide) type and a method for manufacturing the same.
2. Description of the Prior Art
Typical one of conventional semiconductor laser devices has the construction which is shown in FIG. 1. In FIG. 1, the construction thereof is shown in which an epitaxial layer having several thin films is formed on the InP substrate 11. The epitaxial layer is formed by following steps. First, on an n- InP substrate 11 is formed an InGaAsP film 12 which is approximately 0.2 .mu.m in thickness and about 0.95 eV in bandgap composition to form an active film. Next, on the active film 12 is formed a p- InGaAsP film 13 which is approximately 0.2 .mu.m in thickness and about 1.12 eV in bandgap composition to form a photo-waveguide film, and on the photo-waveguide film 13 is formed a p- InP film 14 which is approximately 1.5 to 2 .mu.m in thickness to form a cladding film. Subsequently, on the cladding film 14 is formed a P.sup.+ InGaAs film 15 which is approximately 0.3 to 0.5 .mu.m in thickness to form an ohmic contact film. These films 12, 13, 14, and 15 are formed by an LPE (liquid phase epitaxy) or an MOCVD (metal-organic CVD).
In addition, an etching process is performed using a ridge forming mask to form a ridge of r .mu.m in width. Then, a dielectric layer 16 is deposited on the substrate in which the ridge is formed thus and patterning is performed to form a contact hole of W .mu.m in width, where r is greater than W. A patterned dielectric layer and a patterned photoresist may be used as the ridge forming mask. Also, a p- electrode metal layer 17 is formed on the dielectric layer 16 and in the contact hole, and an annealing is performed. Finally, in order to make cleaving in the laser easily, the InP substrate 11 is ground in the bottom surface to have 100 .mu.m in thickness, and then an n- electrode metal layer 19 is coated on the ground bottom surface.
In the conventional semiconductor laser device which is manufactured by the above described method, since the contact hole has to be formed on upper surface of the ridge, the width W of the contact hole has to be less than the width r of the ridge. Therefore, the ridge has to be relatively largely formed.
For example, when the width W of the contact hole is formed to the extent of 3 .mu.m, the ridge has to be formed having the range of 4 to 5 .mu.m in thickness with respect to align tolerance. This is because mask alignment for forming the contact hole is impossible when the width of the ridge is less than the range. If the width W of the contact hole is formed further large, the ridge also has to formed large in size. As a result, in the case that the laser device formed thus is applied to a communication system, it is particularly effected on high-speed operation in such an optical fiber communication. This technique is disclosed by U.S. Pat. No. 4,888,784.
Also, in the semiconductor laser device as is shown in FIG. 1, threshold current of oscillation thereof as one of principal characteristics of the laser device generally is increased up to 30 mA or more. As a result the characteristic of the laser device is lowered.
If the ridge therein is formed having width of 3 .mu.m or less so as to prevent the characteristic from being lowered, there arises a problem that ohmic contact resistance is largely increased. This is because contact hole has to be formed having width W between 1 .mu.m and 1.5 .mu.m.
In detail, as shown in FIG. 1, the contact area between the p- electrode metal layer 17 and the p- InGaAsP layer 13 is seriously reduced and thus ohmic contact resistance is further increased. Increase in the threshold current of oscillation therein causes due to lowering in the voltage to be applied to p- n junction thereof. As a result, such a semiconductor laser device is increased in heat generation and thus is lowered in average life span thereof. Accordingly, the studies have been developed for methods for manufacturing a semiconductor laser device in which a ridge is further short in width and a contact hole is as long as possible in width thereof.
In order to overcome the above mentioned problems, U.S. Pat. No. 4,830,986 discloses another conventional method for manufacturing the semiconductor laser device as shown in FIGS. 2A and 2B. The method for manufacturing the laser device as shown in FIGS. 2A and 2B has the same method for manufacturing the laser device as shown in FIG. 1 except that a patterned photoresist 26 is used as an etching mask so as to form a ridge on a substrate 21. For example, processing steps for sequentially forming several films 22, 23, 24, and 25 on the substrate 21, as shown in FIG. 2A, are similar to the processing steps for sequentially forming the films 12, 13, 14, and 15 on the substrate 11, as shown in FIG. 1.
As shown in FIG. 2A, in case of forming of a ridge by an under-cut etching method, etching is not performed only in the direction perpendicular to the upper surface of the substrate 21 and is performed in overall directions under the patterned photoresist 26. Then, a dielectric layer 27, as made of SiO.sub.2, is deposited, in the arrow directions A, B, and C as shown in FIG. 2, on the patterned photoresist 26 and the p- InGaAsP layer 23 as well as both side walls of the ridge, using an E-beam deposition as well-known in the art.
If the dielectric material is not formed on a portion of any one of the both side walls, a leak current is largely increased. Accordingly, the depositing process of the dielectric layer 27 has to be completely performed to prevent occurrence of a leakage current therein.
Subsequently, after deposition of the dielectric layer 27, the patterned photoresist 26 is removed by acetone solution as etching solution, and then the dielectric layer on the ridge also is removed. This process is called "lift-off". Finally, a p- electrode metal layer 28 is coated on the dielectric layer 27 and the upper surface of the ridge, and an n- electrode metal layer 29 is coated on the bottom surface of the substrate 21. As a result, the semiconductor laser device having the construction of FIG. 2B is completely manufactured.
Since the semiconductor laser device which is manufactured by the above described method is relatively wide in ohmic contact area, as compared to that of FIG. 1, the ridge can be formed having a relatively short width. As a result, such a semiconductor laser device can be largely reduced in lowering of operating characteristic thereof.
However, since the E-beam deposition is used for formation of the dielectric layer in this method, it is difficult to form dielectric material on both side walls of the ridge. Particularly, interface characteristic between semiconductor layer and the dielectric layer formed by the E-beam deposition is not good, and thus a leak current is generated therebetween.